The present invention is directed to a positive displacement pump for circulating fluids downhole. More particularly, the present invention is directed to a progressing cavity fluid displacement pump for circulating drilling or other fluid downhole in wellbores where normal fluid circulation is either undesirable or impossible (e.g., where the bottomhole pressure is too low). The circulating pump of the present invention is particularly well adapted for use in cased wells to drill out plugs or packers or to remove fill or scale. This circulating pump might also be used in open wells if the hole is sufficiently stable and of sufficiently constant size.
In normal drilling operations, or the like, drilling fluid is circulated from the surface, down the drill string (inside or out) back to the surface (outside or inside the drill string). The drilling fluid performs at least two essential functions:
(1) the fluid functions as a lubricant coolant keeping down the temperature of the bit and the rock surrounding it (i.e., prevents the bit from burning up) and,
(2) the fluid carries the cuttings up the wellbore to the surface removing them from the cutting area (i.e., the drilling fluid reduces the likelihood of sticking the bit in the hole).
In certain drilling/milling operations, or the like, normal circulation of drilling fluid may be impossible or undesirable. Examples of the former include drilling out plugs, packers, etc. or removing fill or scale from a well casing or tubing where there is insufficient clearance between the casing (or the tubing) and the drill string to permit normal circulation, or enlarging the diameter of wellbores where it is not possible to provide closed-loop circulation. An example of circumstances in which circulation would be undesirable might include circulation of acids or other chemicals to remove scale or parafin where normal circulation would be too costly as a result of the amount of fluid required.
In some such situations, the drilling/milling operation is performed without drilling fluid circulation risking burn up and jamming of the tool. Another solution to the problem takes the form of a downhole pump that requires reciprocation of the drill string to effect operation. Such "stroking" of the tubing requires shutdown of the drilling/milling operation and risks sticking the bit in the accumulated cuttings. Further, the valves in this pump are subject to jamming by the cuttings, requiring the entire drill string to be pulled to correct. Lastly, since this pump is operated intermittently, the potential arises for burning up the bit due to the lack of timely stroking.
The present invention overcomes these problems. The present invention utilizes a positive displacement pump (preferably of the progressing cavity type) to circulate drilling fluid downhole. Each end of the rotor has a longitudinally extending straight portion to enable each end of the rotor to be connected by first and second attachment means to an upper element and a lower element, respectively. The second attachment means includes a sliding sprocket to prevent the axial compression of the drill string, that occurs when the drill is engaged, from being transmitted to the rotor (which could potentially cause jamming and/or increased wear on the stator). The cylindrical casing housing the stator is maintained stationary (i.e., does not rotate) by virtue of bow strings engaging the cased (or uncased) wellbore. Rotary force is transmitted from the upper element to the lower element through the stator by the rotor itself.
The cuttings-ladened drilling fluid is pumped upwardly through a check valve into a sediment settling chamber and then out of the drill string through one or more discharge ports to be returned to the lowermost end of said drill string.
Other features, characteristics and advantages of the present invention will become apparent after a reading of the following description.